In production processes, optical measurements can be performed in gases, liquids, solids, and multiphase mixtures to determine certain properties of the production process or about a substance formed as a product of the process. From such measurements, values of measurands correlating to the concentration of educts and/or additives of the process can be determined. For example, in a chemical production process, concentrations of reactants used in the production process and/or the concentration of products produced in the process within a process medium can be monitored, and the process sequence can be controlled and/or regulated based upon the measured data obtained.
The process medium is generally contained in a process container, such as a reactor, a fermenter, a duct or a pipe line. Sensors capable of performing such measurements may be attached to the process container. Alternatively, the process medium may be routed through a flow cell of the sensor to analyze at least a sample from the process container. Such sensors may be referred to as in-line sensors. Conventional in-line sensors are commonly used to evaluate various fluid properties such as concentration through optical measurements of the medium in the process. Such optical measurements may be performed by optical sensors that operate upon absorption of light from a light source in part of optical spectra by the medium as measured by a detector.
Optical sensors, including in-line optical sensors, require periodic calibration to assure required accuracy and dynamic range. One common calibration method uses neutral density (ND) optical filters of known absorption positioned at any point in the optical path between the light source and detector of the sensor. Such ND filters allow only a known portion of the light form the light source, which is associated with a particular absorption (or transmission) value by the detector. After the calibration procedure, the ND filter is moved out of optical path. In certain applications, such ND filters may be traceable calibration filters per the requirements and specifications of the National Institute of Standards and Technology (NIST). Such filters are subject to deterioration with use and exposure to the environment and thus must be re-certified periodically.
U.S. Pat. No. 5,905,271 discloses an in-line optical sensor that can be utilized in a wide variety of applications (e.g. Spectro radiometers, radiometers, colorimeters, turbidimeters, refractometers, and ultrasonic flow and particle sensing devices) for monitoring photometric the properties of a fluid product stream. Such instruments generally have a light source and a detector positioned on opposite sides of the product stream, with the light source directing a beam of light through the product stream to the detector. The light can be in the ultraviolet, visible or infrared spectrums, and the term light is used herein as including all three.
U.S. Pat. No. 6,512,223 discloses a photometric detector assembly which includes a pair of NIST traceable calibration filters enclosed within a sealed housing and means outside the housing for manually moving the filters into and out of the path of a beam between calibration and normal operating positions. This system can be calibrated to a high degree of accuracy, and being in sealed housing and exposed to the beam only on a limited basis, the filters do not need to be cleaned and/or replaced as often as they otherwise would. However, it movement the filters between the calibration and operating positions is a manual operation.
Often, it is not convenient to use a photometric detector such as the one disclosed in U.S. Pat. No. 6,512,223 due to size constraints within the facilities of the process to be monitored or due to cost constraints. In other circumstances, the process may include in-line optical sensors that do not have built-in calibration filters, thus requiring the in-line optical sensors to be removed from the process for calibration and then reinstalled. Accordingly, there remains a need for further contributions in this area of technology.